The present invention relates to an image forming apparatus, for example, a copying machine, a facsimile machine, a printer, or the like, which forms an image with the use of an electrophotographic method or an electrostatic recording method. It also relates to a transfer medium bearing member employed by such an image forming apparatus.
In an image forming apparatus, for example, an electrophotographic image forming apparatus, the peripheral surface of a cylindrical electro-photographic photoconductive member (photoconductive drum) as an image bearing member is uniformly charged, and an electrostatic latent image is formed on the uniformed charged surface in accordance with image formation data. This electrostatic latent image is visualized with the use of developer; a so-called toner image is formed. Then, the toner image is transferred from the photoconductive drum onto a piece of transfer medium (recording medium), and is fixed to the transfer medium, to obtain a copy or a print.
Some of the image forming apparatuses are color image forming apparatuses capable of forming a full-color image as well as a monochromatic image. These color image forming apparatuses can be divided into two groups according to the manner in which a full-color image is formed. In one group, a color image forming apparatus comprises a plurality of image forming stations, each of which has its own photoconductive drum, and in each of which a toner image, which is different in color from the toner image formed in the other stations, is formed on the photoconductive drum. A plurality of the thus formed toner images different in color are consecutively transferred in layers onto the same recording medium borne on a transfer medium bearing member, to form a full-color image. In the other group, a color image forming apparatus comprises only a single image forming station with a single photoconductive drum. In a full-color image forming operation, a plurality of tone images different in color are formed in succession on the same photoconductive drum after the preceding toner image is transferred onto the recording medium borne on a transfer medium bearing member. In these groups of image forming apparatuses, recording medium is conveyed by a transfer bearing member, for example, an endless belt suspended around a plurality of rollers, a cylinder formed by stretching a sheet of specific material around a cylindrical skeletal frame, or the like.
FIG. 3 shows the general structure of an example of a color image forming apparatus. An image forming apparatus 100 comprises a plurality of image forming stations Py, Pm, Pc, and Pk. In each image forming station, a toner image different in color from the toner image formed in the other stations is formed. The toner image formed in each station is consecutively transferred onto the same recording medium to form a color image.
The image forming apparatus comprises a transfer belt 51 as a transfer medium bearing member, which is an endless belt and is suspended around four rollers: a driving roller 52 and three supporting rollers 53a, 53b, and 53c. Located above the transfer belt 51 in this embodiment are four image forming stations Py, Pm, Pc, and Pk for forming yellow, magenta, cyan, and black images, correspondingly. Since the four image forming stations Py, Pm, Pc, and Pk are the same in structure, the structures of the image forming stations will be described in detail with reference to the image forming station Py for forming a toner image of a first color (yellow). In the drawings, the elements in each image forming station, which are the same in function as those in the other stations, are given the same referential codes, but are differentiated from those in the other stations by addition of subscripts y, m, c, and k, correspondingly to the referential codes Py-Pk for the yellow, magenta, cyan, and black image forming stations.
Referring to FIG. 4, the image forming station Py for the first color has a cylindrical photoconductive member (photoconductive drum) 1y as an image bearing member. During an image forming operation, the photoconductive drum 1y is rotationally driven in the direction indicated by an arrow mark A by a driving means (unshown), and the peripheral surface of the photoconductive drum 1y is uniformly charged by a magnetic brush type charging apparatus as a charging means. Then, the charged photoconductive drum 1y is exposed to an image exposure light L representing the yellow component of an original, by an exposing apparatus (LED based scanning apparatus) 3y. As a result, an electrostatic latent image in accordance with the inputted image formation data is formed on the peripheral surface of the photoconductive drum 1y. Next, the electrostatic latent image on the photoconductive drum 1y is developed into a yellow toner image by a developing apparatus 4y. 
At the same time as the yellow toner image on the photoconductive drum 1y reaches a transfer nip between the peripheral surface of the photoconductive drum 1y and a transfer belt 51, a recording medium P, for example, a piece of recording paper, which is fed into the image forming apparatus main assembly from a recording medium cassette 80 as a recording medium storage by a sheet feeding roller 81 or the like, is delivered to the transfer nip by a registration roller 82. In the transfer nip, electrical charge, which is opposite in polarity to the toner, is applied to the recording medium P, on the reverse side, that is, the side on which the image is not going to be transferred and is in contact with the transfer belt 51, by a transfer charge blade 54 as a transfer charging device charged with transfer bias. As a result, the toner image on the photoconductive drum 1y is transferred onto the transfer medium P, on the top side. A transferring apparatus 5 (belt type transferring apparatus) comprises the transfer belt 51, rollers 52, 53a, 53b, and 53c, and transfer charge blades 54y-54k. 
After the transfer of the yellow toner image onto the recording medium P, the recording medium P is conveyed to the image forming station Pm for a second color (magenta), as the transfer belt 51 moves in the direction indicated by an arrow mark f.
The image forming station Pm for the second color is the same in structure as the image forming station Py for the first color. Thus, the same processes as those carried in the image forming station Py are carried out in the image forming station Pm. That is, a latent image is formed on the photoconductive drum 1m, and the magenta developing apparatus 4m develops the latent image into a magenta toner image with the use of magenta toner. Then, the magenta toner image is transferred onto the recording medium P, in a manner to be layered on the yellow toner image, by the function of the transfer charge blade 54m, in the transfer nip.
Next, a cyan toner image and a black toner image are formed in the image forming stations Pc for a third color and the image forming station Pk for a fourth color, respectively, and are transferred onto the recording medium P by the transfer charge blades 54c and 54k, in a manner to be layered on the preceding two toner images, in the corresponding image forming stations. Consequently, a color image, or a composite of four layers of toner images different in color, is formed on the recording medium P. At this point, the color image is yet to be fixed.
After the transfer of the four toner images onto the recording medium P, the recording medium P is conveyed to a fixing apparatus 6 which comprises a fixing roller 6a containing a heating means, and a driving roller 6b. In the fixing apparatus 6, the toner images on the recording medium P are fixed, as a permanent full-color image, to the surface of the recording medium P by the application of heat and pressure by the fixing roller 6a and driving roller 6b. After the fixation of the toner images, the recording medium P is discharged into an external delivery tray (unshown), or the like, of the image forming apparatus.
After the recording medium P is separated from the transfer belt 51, the transfer belt 51 is removed from the electrical charge on the reverse side, by a combination of a grounded electrically conductive fur brush 11 and a grounded transfer belt driving roller 52. Further, the foreign substances, for example, toner particles (residual toner particles), paper dust, and the like, on the transfer belt 51, are removed by a transfer belt cleaner 12 comprising a urethane rubber blade and the like, to be prepared for the next image formation cycle.
On the portion of each of the photoconductive drums 1y-1k, which has just passed the transfer nip, residual toner particles, that is, toner particles which failed to be transferred onto the recording medium P, are present, although only by a small amount. These residual toner particles are scraped away, electrostatically and mechanically, and are temporarily absorbed, by the magnetic brush of each of the magnetic brush type charging apparatuses 2y-2k. As the amount of the transfer residual toner particles in the magnetic brush of each of the magnetic brush type charging apparatuses 2y-2k increases, the electrical resistance of the magnetic brush itself increases, and eventually, the magnetic brush fails to sufficiently charge the photoconductive drum. As a result, difference in electrical potential is created between the magnetic brush and the peripheral surface of the photoconductive drum, causing the transfer residual toner particles in the magnetic brush to electrostatically transfer onto the photoconductive drum. After transferring onto the photoconductive drum, the transfer residual toner particles are electrostatically taken into the developing apparatus, to be consumed during the following image formation cycles.
In the above-described image forming apparatus 100, the toner images formed in the image forming stations Py, Pm, Pc, and Pk must be precisely aligned, and therefore, the transfer belt 51 as a transfer medium bearing member, which holds and conveys the transfer medium P, must be stable. In the image forming apparatus 100 in this embodiment, the recording medium P is electrostatically held to the transfer belt 51 with the use of electrostatic adhesion rollers 55 and 56. The electrostatic adhesion roller 56 is grounded. As the recording medium P enters an electrostatic adhesion nip in which the electrostatic adhesion rollers 55 and 56 oppose each other with the interposition of the transfer belt 51, a positive bias of 1 kV is applied to the electrostatic adhesion roller 55 to electrostatically adhere the recording medium P to the transfer belt 51.
The above described electrostatic adhesion of the recording medium P, and the toner image transfer in each of the image forming stations Py, Pm, Pc, and Pk, are significantly affected by the electrical properties (electrical resistance, dielectric constant, and the like) and mechanical properties (thickness, mechanical strength, surface properties, and the like) of the transfer belt 51.
First, regarding the electrical properties of the transfer belt 51, for example, electrical resistance, if the electrical resistance of the transfer belt 51 is lower than a certain level, the biases applied to the transfer charge blade 54 and electrostatic adhesion roller 55 interfere with each other through the transfer belt 51, and the electrical charge given to the transfer belt 51 by the transfer charge blade 54 and electrostatic adhesion blade 55 is likely to attenuate. As a result, toner images are disturbed after they are transferred onto the recording medium P, and the electrostatic force for keeping the recording medium P adhered to the transfer belt 51 weakens.
On the other hand, if the electrical resistance of the transfer belt 51 is higher than a certain level, the absolute values of the biases applied to the transfer charge blade 54 and electrostatic adhesion roller 55 must be greater, which is likely to trigger abnormal electrical discharge in the transfer nip and electrostatic adhesion nip, and the abnormal electrical discharge results in an image of inferior quality.
Next, regarding mechanical properties, for example, thickness, if the thickness of the transfer belt 51 is less than a certain level, the transfer belt 51 is insufficient in mechanical strength, being likely to break and/or stretch, and therefore, is not stable, whereas if the thickness of the transfer belt 51 is more than a certain level, the absolute values of the biases applied to the transfer charge blade 54 and electrostatic adhesion roller 55 must be greater than they must be if the electrical resistance of the transfer belt 51 is higher than a certain level, rendering the transfer belt 51 unsatisfactory.
In other words, the transfer belt 51 is sometimes required to satisfy two mutually contradictory requirements, even regarding only one of the aforementioned physical properties. As one of the solutions to this problem, a multilayered transfer belt (51) disclosed in Japanese Laid-open Patent Application 2-148074 is frequently used. This patent application proposes that various functions of the transfer belt (51) be divided among the plurality of functional layers. More specifically, in order to prevent the transfer belt from failing to be satisfactorily removed of the electrical charge thereon, while providing the transfer belt with a sufficient amount of mechanical strength, the transfer belt is multilayered; it is provided with a surface layer, the electrical resistance of which has been adjusted to a sufficiently low level, and a base layer which is mechanically strong.
However, when a transfer belt 51 having a plurality of layers different in function is employed, the transfer belt 51 sometimes warps as shown in FIG. 11, which shows the widthwise cross section of the transfer belt 51 as seen from the direction to which the transfer belt 51 advances. As is evident from the drawing, the belt 51 sometimes warps at both edges.
The studies made by the inventors of the present invention revealed that this phenomenon, or the warping, was caused by the difference in the coefficient of linear expansion among the plurality of functional layers. More specifically, the warping of the transfer belt 51 occurs when the plurality of layers formed of resinous material are different in the ratio at which their measurements fluctuate due to either or both of the ambient temperature and humidity of the transfer belt 51.
If warping such as the above described occurs to the belts or sheets, for example, the transfer belt 51 as a transfer medium bearing member, which are involved in the image forming processes within the image forming apparatus 100, the belts or sheets fail to uniformly contact their counterparts. For example, the transfer belt 51 fails to uniformly contact the photoconductive drum 1 with the interposition of the recording medium P, in the transfer nip, causing the transfer charging means to fail to uniformly charge the transfer belt 51, and further, a gap is created between the recording medium P and transfer belt 51, along the both edges of the recording medium P in terms of the widthwise direction of the transfer belt 51 as shown in FIG. 12. As a result, the toner images are improperly transferred, resulting in a full-color image of inferior quality.
Thus, the primary object of the present invention is to prevent the transfer medium bearing member employed by an image forming apparatus, from suffering from deformation such as warping caused by the changes in the environmental factors such as temperature or humidity.
Another object of the present invention is to provide an image forming apparatus capable of always producing an excellent image, more specifically, an image which does not suffer from defects which result from unsuccessful image transfer, by preventing the transfer medium bearing member from suffering from deformation such as warping caused by the changes in the environmental factors such as temperature or humidity.
According to an aspect of the present invention for achieving the above objects, a transfer medium bearing member for holding and conveying a transfer medium onto which an image on an image bearing member is to be, or has been, transferred, comprises a minimum of first and second layers laminated to each other, and the amount Xa of the change in the length of the first layer, the amount Xb of the change in the length of the second layer, the thickness Ha of the first layer, and thickness Hb of the second layer, satisfy the following inequity;
|Xaxe2x88x92Xb| less than Ha+Hb.
According to another aspect of the present invention, in an image forming apparatus comprising: an image forming means for forming an image on an image bearing member; a transfer medium bearing member for holding and conveying a transfer medium; and a transferring means for transferring an image on the image bearing member onto the transfer medium being held and conveyed by the transfer bearing member, the transfer medium bearing member comprises a minimum of first and second layers laminated to each other, and the amount Xa of the change in the length of the first layer, the amount Xb of the change in the length of the second layer, the thickness Ha of the first layer, and thickness Hb of the second layer, satisfy the following inequity:
|Xaxe2x88x92Xb| less than Ha+Hb.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.